Glacial Forcing Research Articles

Climate sensitivity estimated from ensemble simulations of glacial climate

The concentration of greenhouse gases (GHGs) in the atmosphere continues to rise, hence estimating the climate system’s sensitivity to changes in GHG concentration is of vital importance. Uncertainty in climate sensitivity is a main source of uncertainty in projections of future climate change. Here we present a new approach for constraining this key uncertainty by combining ensemble simulations of the last glacial maximum (LGM) with paleo-data. For this purpose we used a climate model of intermediate complexity to perform a large set of equilibrium runs for (1) pre-industrial boundary conditions, (2) doubled CO2 concentrations, and (3) a complete set of glacial forcings (including dust and vegetation changes). Using proxy-data from the LGM at low and high latitudes we constrain the set of realistic model versions and thus climate sensitivity. We show that irrespective of uncertainties in model parameters and feedback strengths, in our model a close link exists between the simulated warming due to a doubling of CO2, and the cooling obtained for the LGM. Our results agree with recent studies that annual mean data-constraints from present day climate prove to not rule out climate sensitivities above the widely assumed sensitivity range of 1.5–4.5°C (Houghton et al. 2001). Based on our inferred close relationship between past and future temperature evolution, our study suggests that paleo-climatic data can help to reduce uncertainty in future climate projections. Our inferred uncertainty range for climate sensitivity, constrained by paleo-data, is 1.2–4.3°C and thus almost identical to the IPCC estimate. When additionally accounting for potential structural uncertainties inferred from other models the upper limit increases by about 1°C.

Astronomical timescale and palaeoclimatic implication of stacked 3.6-Myr monsoon records from the Chinese Loess Plateau

Magnetic susceptibility and grain size records from two continuous red-clay and loess–palaeosol sequences on the Chinese Loess Plateau have been generated to investigate the evolution and variability of the East Asian monsoon (EAM) during the late Pliocene and Pleistocene. Tuning the grain size records to orbital obliquity and precession yields an improved astronomical timescale for the loess–palaeosol sequence (0–2.6 Ma), and an extended age model for the upper red-clay formation (2.6–3.6 Ma). This timescale indicates older ages for a number of the magnetic polarity boundaries, consistent with lock-in depth offsets in loess sequences. Good site-to-site correlation enables generation of stacked grain size and susceptibility records spanning the last 3.6 Myr. These records indicate that monsoon evolution since the late Pliocene can be subdivided into three phases: 0–1.25, 1.25–2.72 and 2.72–3.4 Ma; each phase is characterized by unique amplitude and frequency characteristics for both summer monsoon (magnetic susceptibility) and winter monsoon (grain size). Spectral analyses of the stacked monsoon proxies indicate that characteristics of both the summer and winter monsoons are dominated mainly by variance in the eccentricity (410- and 100-kyr), obliquity (41-kyr) and precession (23- and 19-kyr) bands over the past 3.4 Myr, implying a non-linear response of the long-term EAM evolution to orbital and glacial forcing.

Linear or non-linear rheology in the Earth's mantle: the prevalence of power-law creep in the postglacial isostatic readjustment of Laurentia

SUMMARY The great majority of postglacial rebound computations carried out during the last three decades assumed a purely linear rheological relation for the mantle. Experimental data on high-temperature creep deformation and modelling of other tectonic processes, however, might also support the existence of non-linear creep mechanisms. We addressed postglacial rebound in North America through an axially symmetric finite-element model with a composite (linear plus non-linear) mantle rheology. In such a formulation, the transition stress σ T governs the balance between linear and non-linear creep components, while the term σ B, added to the effective shear stress, accounts for the background (ambient) stress induced by convection and other tectonic processes. By varying σ T and σ B in the ranges 0‐10 MPa and 0‐5 MPa respectively, we found that composite models fit Relative Sea Level (RSL) variations at 29 North American sites better than the purely linear model. On the basis of the effective shear stress induced in the mantle by glacial forcing (1‐3 MPa), our results indicate that power-law creep accounts for the majority of the strain rate.

The importance of precessional signals in the tropical climate

Past research on the climate response to orbital forcing has emphasized the glacial-interglacial variations in global ice volume, global-mean temperature, and the global hydrologic cycle. This emphasis may be inappropriate in the tropics, where the response to precessional forcing is likely to be somewhat independent of the glacial-interglacial variations, particularly in variables relating to the hydrologic cycle. To illustrate this point, we use an atmospheric general circulation model coupled to a slab ocean model, performing experiments that quantify the tropical climate’s response to (1) opposite phases of precessional forcing, and (2) Last Glacial Maximum boundary conditions. While the glacially-forced tropical temperature changes are typically more than an order of magnitude larger than those arising from precessional forcing, the hydrologic signals stemming from the two forcings are comparable in magnitude. The mechanisms behind these signals are investigated and shown to be quite distinct for the precessional and glacial forcing. Because of strong dynamical linkages in the tropics, the model results illustrate the impossibility of predicting the local hydrologic response to external forcing without understanding the response at much larger spatial scales. Examples from the paleoclimate record are presented as additional evidence for the importance of precessional signals in past variations of the tropical climate.

Tropical Cooling at the Last Glacial Maximum: An Atmosphere–Mixed Layer Ocean Model Simulation

The sensitivity of tropical temperature to glacial forcing is examined by using an atmosphere–mixed layer ocean (A–MLO) model to simulate the climate of the last glacial maximum (LGM) following specifications established by the Paleoclimate Modeling Intercomparison Project. Changes in continental ice, orbital parameters, atmospheric CO2, and sea level constitute a global mean radiative forcing of −4.20 W m−2, with the vast majority of this forcing coming, in nearly equal portions, from the changes in continental ice and CO2. In response to this forcing, the global mean surface air temperature decreases by 4.0 K, with the largest cooling in the extratropical Northern Hemisphere. In the Tropics, a more modest cooling of 2.0 K (averaged from 30°N to 30°S) is simulated, but with considerable spatial variability resulting from the interhemispheric asymmetry in radiative forcing, contrast between oceanic and continental response, advective effects, and changes in soil moisture. Analysis of the tropical energy balance reveals that the decrease in top-of-atmosphere longwave emission associated with the tropical cooling is balanced primarily by the combination of increased reflection of shortwave radiation by clouds and increased atmospheric heat transport to the extratropics. Comparisons with a variety of paleodata indicate that the overall tropical cooling is comparable to paleoceanographic reconstructions based on alkenones and species abundances of planktonic microorganisms, but smaller than the cooling inferred from noble gases in aquifers, pollen, snow line depression, and the isotopic composition of corals. The differences in the magnitude of tropical cooling reconstructed from the different proxies preclude a definitive evaluation of the realism of the tropical sensitivity of the model. Nonetheless, the comparisons with paleodata suggest that it is unlikely that the A–MLO model exaggerates the actual climate sensitivity. The similarity between the sensitivity coefficients (i.e., the ratio of the change in global mean surface air temperature to the change in global mean radiative forcing) for the LGM simulation and a simulation of CO2 doubling suggests that similar climate feedbacks are involved in the responses to these two perturbations. More comprehensive simulation of the tropical temperature sensitivity to glacial forcing will require the use of coupled models, for which a number of technical obstacles remain.

Mantle viscosity inferences from joint inversions of pleistocene deglaciatio‐induced changes in geopotential with a new SLR analysis and polar wander

Joint inversions of post‐glacial rebound‐induced secular changes in low degree harmonics of the geopotential and true polar wander (TPW) systematically indicate that the upper mantle viscosity has a value that is considerably smaller than 1021 Pas. The lower mantle viscosity is about one order of magnitude larger than the upper mantle viscosity. Discrepancies in the viscosity profiles inferred from even and odd zonals are likely an indication of recent, unmodeled glacial forcings and/or insufficient precision in the evaluation of the mass loss from Pleistocene ice sheets. The influence of systematic errors in SLR data reduction due to thermal drag on the LAGEOS satellites for the uneven zonals of the geopotential is avoided by making use of the Starlette and Stella satellites to recover the uneven zonals and by considering lumped coefficients for these uneven terms.

Application of Ocean Models for the Interpretation of Atmospheric General Circulation Model Experiments on the Climate of the Last Glacial Maximum

We examined the response of our ocean models of physical circulation and geochemical tracer distributions on atmospheric glacial forcing. The CLIMAP Project Members (1981) sea surface temperature (SST) was taken directly as a boundary condition. The wind stress and freshwater flux were derived from the ice age response of an atmospheric general circulation model (AGCM). Near the surface, the ocean response in temperature and circulation reflects primarily the imposed glacial forcing fields. The simulated deep ocean response, especially in the North Pacific, is in conflict with the observational evidence. The glacial changes in the physical ocean circulation appear to be qualitatively as derived from sediment cores, but, probably, highly overestimated. The misfit between model and data is established clearly in the distribution of δ13C, where a direct comparison with sediment core data is possible. The simulated AGCM freshwater flux, which is linked for example by the evaporation to the imposed SST field, is suspected to be the most probable reason for this conflict.

Mantle rheology and satellite signatures from present‐day glacial forcings

We study the changes of the long‐wavelength portion of the Earth's gravity field as a result of present‐day glacial discharges and the possible growth of the Antarctic ice sheet. We employ both Maxwell and Burgers' body rheologies and find that there are significant differences in the responses between the two rheologies for time spans less than a century. Both present‐day glacial forcing and the ice buildup on Antarctica can cause non‐negligible perturbations in j2 of about one‐third the observed amount. The contributions from the two mechanisms have opposite signs in j2. Only for l = 3 and 6 do these cryospheric contributors act in concert with one another in disturbing the gravity field. All other degree harmonics, up to l = 9, have opposite signs from these two excitations. For a steady‐state lower‐mantle viscosity of 1022 Pa s, j3 can attain a maximum value of around 1.5×10−11 yr−1 and j6 a value of −1.3×10−11 yr−1 for current forcings. These have signs opposite to those predicted for Pleistocene deglaciation. All harmonics, up to l = 9, are sensitive to variations of the steady‐state viscosity in the lower mantle, even for time scales shorter than 100 years. It is proposed that detailed satellite monitoring of present‐day ice movements in conjunction with geodetic satellite missions will eventually provide a viable alternative for estimating deep mantle viscosity, useful in mantle convection models.

Excitation of the Earth's rotational axis by recent glacial discharges

We study the effects of present‐day glacial discharges and the growth of the Antarctic ice sheet on exciting the Earth' rotational axis. Glacial forcing could cause a maximum change in J2 of about one‐third of the observed amount, for the Maxwell rheology and for Burgers’ body models with a long‐term, lower‐mantle viscosity greater than about 1023P. For transient rheologies the amount of excitation due to glacial melting decreases. Polar wander is not much excited by recent glacial melting for the various types of rheologies examined.

Viscosity of the lower mantle as inferred from rotational data

Until recently most estimates of the deep mantle viscosity were based on analyses of postglacial uplifts, and there were no serious estimates of the sensitivity of the derived viscosity solutions to variations of the input parameters, such as those associated with the surface loading. In this paper the viscosity of the lower mantle is arrived at by making comparisons of the observed secular motions of the earth's rotation axis with theoretical results from a layered viscoelastic, rotating earth, which has been subjected to glacial forcing. Our model, consisting of an elastic lithosphere, a two‐layer, adiabatically stratified viscoelastic mantle, and an inviscid core, is essentially analytical, and this makes it economically feasible to use as an aid in conducting an extensive sensitivity analysis of the lower mantle viscosity from changes in the parameters connected with the deglaciation phenomenon. Both sets of rotational data, polar wander and nontidal deceleration of the length of the day, have been employed to constrain the viscosity structure. On the basis of this type of investigation, the viscosity of the lower mantle is found invariably to be larger than that of the upper mantle and lies between 1 and 4×1022 P. Our results, which just rely on the second degree harmonic of the strain field, also lead to a determination of an upper bound to the thickness of the globally averaged lithosphere. This value, ranging between 130 and 190 km, suggests that strong lateral heterogeneities between oceanic and continental plates may extend a few hundred kilometers into the upper mantle.